A known type of heat transfer fluid heater conducts the fluid through a helical tubular coil which has its axis horizontally disposed. The coil may define a furnace chamber or it may be located in such chamber. In any case, the fluid absorbs heat as it flows through the coil under the influence of a pump. By suitable piping, the outlet of the coil may be connected to a remotely situated heat exchanger which yields heat to anything one desires to heat. Typical uses are for supplying heat to chemical process equipment such as reactors, autoclaves, distillation columns, reboilers, mixers and dryers. Other examples of uses are to supply heated fluid to coils for de-icing or simply to heat space.
Usually the temperature requirements of the process in which the heat is yielded dictates the nature of the fluid that is chosen for transferring heat. For high temperature applications, fluids derived from petroleum are usually used because some are available which have desirable properties of high heat transfer coefficients, high boiling points, good thermal capacity and lack of corrosiveness. However, at high temperatures petroleum derivatives and other organic heat transfer fluids deteriorate through either cracking or oxidation. Sometimes volatile hydrocarbons are produced which may cause vapor lock in the system. Cracking also produces nonvolatile materials or polymers which increase fluid viscosity and cause deposition of coke or carbon on the inside of the heater coil surfaces. The deposits have an insulating quality which often results in the tubes becoming overheated such that premature failure occurs. One approach to ridding the tubing of deposits is to force pistons known as "pigs" through the coil for scraping off the sediment. This is disadvantageous because it requires the system to be opened periodically.
Another problem with horizontal tube fluid heaters is that of purging the fluid system of water vapor, air and other gases. One source of water results from the need to make a hydrostatic pressure test on the system during manufacture at the factory or on site after some maintenance has been performed. Despite efforts to blow the water out, there is always some residual that settles in the lower parts of the loops of the tubular helix. Moreover, there are occasions when the fluid heater must be inactivated for a period of time and this calls for forcing all of the fluid out of the coil. Unfortunately, there is always some residual fluid remaining in the lower portion of the coil loops and, heretofore, there has been no satisfactory way of removing it or draining it. Another problem is that when gases and vapors rise to the upper portions of the coil loops, the absence of fluid in such portions results in localized overheating of the coil and accelerated degradation of the residual fluid.